Modified rolled thread form for studs

ABSTRACT

A stud for securing a cylinder liner to a head has a modified thread form engaging a threaded hole in the stud with an interference fit. The thread is rolled and all threads have the same pitch diameter except the endmost thread. Three relief channels are formed in the threads and extend axially from the stud end through several threads while some full threads remain to engage the hole. The relief channels decrease hoop stress in the threaded hole and allow debris developed during installation to escape from the thread interface to prevent entrapment therein.

TECHNICAL FIELD

This invention relates to thread forms and particularly to thread formsfor studs used in cylinder liners.

BACKGROUND

Cylinder liner assemblies used in large diesel engines include waterpassages coupled to a head. The head is secured to the liner assembly byseveral studs which are fastened to the liner. The liner has a threadedhole to receive a threaded end section of each stud. To assure that thestuds remain securely fastened to the liner so that the stud does notmove when a nut is applied or removed from the outer end, aninterference fit is used in the threaded connection. Also, to preventthe leakage around the stud of water entering the threaded hole throughporous liner walls, full thread engagement of the outer threads with thehole is required. The ANSI standard B1.12 "Class 5 Interference-FitThread" sets forth the standard specifications for such studs.

When using the standard studs with rolled threads, it was found that theinterference fit caused spalling believed to be due to welding of theinterface surfaces as a result of high frictional forces. The frictionis caused by the geometric interference, micro finish of the matingthreads, and debris trapped between the mating threads duringinstallation. During installation of a class 5 thread, material isshaved off or spalls off the mating threads and becomes trapped in thethread interface. This trapped debris increases the thread interferenceand gouges the threads creating larger balled up debris. If the debrisacquires sufficient size, the friction and driving torque increase untilfailure occurs. The installation of the standard studs was marked byhigh torque and failures such as stripping the thread form on both themale and female mating members, cracking of the female threaded memberdue to excessive hoop stress, torsional failure of the fastener throughthe minor diameter of the thread form, and stalling of the fastenerbefore the fastener is driven to the proper depth.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a modifiedinterference-fit thread form having lower friction force duringinstallation while maintaining a high breakaway torque. It is anotherobject of the invention to provide such a modified thread form whichminimizes the entrapment of debris in the thread interface.

The invention is carried out by a stud for engaging a threaded hole withan interference fit having: a threaded end section with rolled threads,all of the threads except the endmost thread having uniform diameters,including a uniform pitch diameter adapted for interference with thethreaded hole; a plurality of interruptions in several threads nearestthe end, the remaining threads being full threads; each interruptioncomprising a locally reduced maximum diameter of the several threads toa value less than the said pitch diameter to form a recess which issymmetrical with respect to a radial plane of the stud; and the minimumdiameter of the stud in the threaded section being defined by the minorthread diameter, whereby the said recess is not deeper than the minorthread diameter.

BRIEF DRAWING DESCRIPTION

The above and other advantages of the invention will become moreapparent from the following description taken in conjunction with theaccompanying drawings wherein like references refer to like parts andwherein:

FIGS. 1 and 2 are side and end views, respectively, of a stud having amodified thread form, according to the invention;

FIGS. 3 and 4 are side and end views, respectively, of a blank used tomake the stud of FIGS. 1 and 2;

FIG. 5 is an isometric view of a cylinder liner with some studsinstalled;

FIG. 6 is a cross section of mating thread forms according to theinvention; and

FIG. 7 is a graph showing torque curves for a standard rolled thread anda modified thread formed according to the invention.

DETAILED DESCRIPTION

The ensuing description is directed to a modified thread form which wasdeveloped for cylinder liner studs to prevent breakage of liners andstuds and also to provide a seal in the event of water leakage from theliner to the tapped hole receiving the stud. Similar advantages may beobtained in other stud applications, especially regarding stud or nutbreakage. Some other potential applications are for studs to hold thefuel injector in place on a cylinder head, studs to clamp the cylinderhead to the block, and studs to secure rocker arms to cylinder heads.

Referring to FIGS. 1 and 2, a stud 10 with ends 12 and 14 has a shank16, a standard threaded section 18 adjacent the end 12 and a modifiedthreaded end section 20 adjacent the end 14. The threaded section 20 hasat least three full threads 22 adjacent the shank and severalinterrupted threads 24 adjacent the end 14. The threads are all of equaldiameter except for the endmost thread 28 adjacent the end 14 which is atypical starting thread and is gradually reduced in diameter tofacilitate starting the stud in a nut. A chamfer 27 is formed at the end14.

Each interrupted thread has three gaps equally spaced around the stud toform three axial recesses or channels 26 forming relief zones extendingfrom the endmost thread 28 adjacent the end 14 to the beginning of thefull threads. Preferably the channel length is 50 to 60% of the lengthof the threaded section 20. The bottom 30 of each channel 26 isgenerally flat and the sides are sloped due to the ramped ends 32 of theinterrupted threads 24. The ends are sloped at an angle of about, say,30° with respect to the bottom 30. Each channel 26 is symmetrical withrespect to a radial plane 25 of the stud and is normal to the plane 25.The bottom 30 of the channel is at or above the core defined by theminor diameter of the thread so that the channel does not diminish theminimum cross section of the threaded section which is established bythe thread. Thus the center of the bottom 30 is tangent to the core orparallel to the tangent and a few thousandths of an inch above thetangent. The bottom 30 of the channel must be somewhat below the pitchdiameter to afford relief from the thread interference.

The end of the blank 34 used to make the stud is shown in FIGS. 3 and 4.The blank comprises a shank 16, a reduced diameter end section 20', achamfer 27 and three equally spaced flats 26' adjacent the end. Eachflat is symmetrical with respect to a radial plane 25 and normalthereto. The width of the flat is typically approximately equal to thenominal radius of the stud or larger, i.e. a 3/4 inch stud would have ablank flat width of about 3/8 inch. The end section 20' is cylindricalbut for the flats 26' and the chamfer 27, so that the threaded productwill have threads of uniform major, minor and pitch diameters except forthe endmost thread 28. The threads are formed by rolling. As a result ofthe rolling operation, the ends of the threads 24 are raised above theflats 26' to form the sloped ends of the threads at the sides of thechannel 26. The resultant gap between the thread ends will be about 1/4inch for the 3/4 inch stud example.

A cylinder liner 40 having threaded holes 42 for receiving the studs 10is shown in FIG. 5, while FIG. 6 shows a cross section of a stud 10 witha modified thread end section 20 engaged in the threaded hole 42 orother nut. The threads 22 and 24 all make interfering engagement. Thechannels 26 provide a relief between the interrupted threads 24 to allowany debris developed during stud installation to remain clear of thethreads 24 or even to escape to the bottom of the hole, therebyminimizing the entrapment of debris in the thread interface where theexternal threads 22, 24 engage the internal threads of holes 42. Asecond effect of the channels 26 is that they help relieve the hoopstress in the liner or nut 42. The diagram of FIG. 7 shows the drivingtorque of a full rolled thread in a broken line and of the modifiedrolled thread in a solid line, both threads having the same diameters.Typically the maximum torque is reduced by 25% and the rate of torqueincrease is more uniform for the modified thread compared to theconventional full thread.

The preferred dimensions of the modified stud and the correspondingthreaded hole are slightly changed from the standard ANSIspecifications. For a 3/4-10 rolled thread on the stud and acorresponding cut thread in the nut or liner, the following table givesthe thread diameters in inches.

    ______________________________________                                                    Internal thread                                                                          External Thread                                        ______________________________________                                        Major Diameter                                                                              0.7360-0.7290                                                                              0.7500 min                                         Pitch Diameter                                                                              0.6935-0.6910                                                                              0.6860-0.6895                                      Minor Diameter                                                                              0.6378 max   0.645-0.662                                        ______________________________________                                    

With this thread size, the flats 26' on the blank extend 60% of theaxial length of the threaded section 20 and are 0.015 inch less deepthan the minor diameter of the thread so as not to decrease the minimumcross section of the stud. There are at least three full threads 22engaged with the internal threads of the hole 42. The three full threads22 carry 80% of the tensile load thereby providing this thread form withthe equivalent tensile load handling capabilities of a full thread form.The relief zones or channels 26 provide cavities where the debris formedduring installation can escape from the thread interface to prevent highlocalized interference. Hoop stress in the mating female member 42 isdecreased 35% by decreasing the nut deflection at the relief zones.Although the thread is not designed as a self-tapping thread, thestarting portion of the modified thread form acts essentially as a tapbefore the full thread form engages the threaded hole 42. The spalling,which normally occurs with a class 5 fit, takes place in the first fewthreads which have a relief to accept the debris, and the internalthreads are sufficiently enlarged that only little further galling takesplace between the full threads and the nut.

The advantages resulting from these features include easier installationwith fewer failures of the studs as well as the liners or other femaledevices while maintaining the high breakaway torque so that the studdoes not move when a nut is installed on or removed from the other endof the stud.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A stud for engaging athreaded hole in a cylinder liner having:a shank, an end, and a threadedsection adjacent the end; the threaded section having a plurality offull threads adjacent the shank and a plurality of axially extendingrelief zones extending from the full threads to the end for interruptingthreads adjacent the end; the full and partial threads having the sameuniform major and pitch diameters and a minor diameter defining thesmallest cross section of the threaded section; and the relief zoneslying outside the minor diameter of the threads.
 2. A stud for engaginga threaded hole with an interference fit having:a threaded end sectionwith rolled threads, all of the threads except the endmost thread havinga uniform major diameter and a uniform pitch diameter adapted forinterference with the threaded hole; a plurality of interruptions inseveral threads nearest the end, the remaining threads being fullthreads; each interruption comprising a locally reduced maximum diameterof the several threads to a value less than the said pitch diameter toform a recess which is symmetrical with respect to a radial plane of thestud; and the minimum diameter of the stud in the threaded section beingdefined by the minor thread diameter, whereby the said recess is notdeeper than the minor thread diameter.
 3. The invention as defined inclaim 2 wherein each said recess has a substantially flat bottom portionwhich is normal to the said radial plane.
 4. The invention as defined inclaim 2 wherein the minor thread diameter defines a core of the threadedsection, and each said recess has a substantially flat bottom regionwhich is substantially tangent to the said core.
 5. A stud for engaginga threaded hole with an interference fit having:a threaded end sectionwith rolled threads, all of the threads except the endmost thread havinga uniform pitch diameter adapted for interference with the threadedhole, whereby debris is produced upon assembly of the stud to thethreaded hole; means for receiving said debris to prevent entrapment ofdebris between the interfering threads comprising a plurality ofchannels axially extending through several threads adjacent the end andabove the minor diameter of such threads; and each channel beingsymmetrical with respect to a radial plane of the stud.
 6. The inventionas defined in claim 5 wherein each channel has a substantially flatbottom approximately tangent to the root of the threads, and is boundedby sloped thread ends on either side of the flat bottom.
 7. Theinvention as defined in claim 5 wherein the rolled threads of the endsection of the stud and the threads of the associated threaded holeapproximate the ANSI standard B1.12 "Class 5 Interference-Fit Thread" of3/4-10 size modified to have a diametral interference at the pitchdiameter substantially in the range of 0.0015-0.0075 inches.